1. Field of Invention
The field of the present invention relates in general to modems and more particularly digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections to high speed back bone networks developed by telecommunications companies. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete XDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrier less AM/PM (CAP).
Each new XDSL protocol raises the bandwidth requirements of subscriber lines. As the bandwidth requirements increase so too does the complexity of the modem components. Additionally, because of the enormous variation in loop loss in the individual subscriber lines to which the modem may be coupled the individual components of the modem transmit and receive path must be reconfigurable to match the available bandwidth on a selected subscriber line. For modems which implement the VDSL protocol for example, the length and quality of the subscriber line will determine whether all or a portion of the two upstream and two downstream communications ranges will be available. The sheer length of the line results in significant attenuation of the upper communication ranges rendering their use impractical. Even on shorter loops the presence of bridges and taps can also constrain bandwidth. In each instance it is critical during the line qualification phase to be able to inexpensively and quickly determine insertion power loss into the subscriber line to which the modem is coupled.
What is needed is a modem with improved capabilities for determining insertion loss into a subscriber line.
The current invention provides a method and apparatus for characterizing a wireline. This includes determination of electrical distance, insertion loss, and gain scaling. The apparatus may be incorporated into physical or logical modems. The method may be implemented with any modulation protocol but is particularly suited for multi-carrier modulation protocols such as discrete multi-tone (DMT).
In an embodiment of the invention an apparatus for wireline characterization in a modem with a transmit path and a receive path each configured to couple with a remote modem via a wireline in which available communication bandwidth is split into a number of independent sub-channels is disclosed. The apparatus comprises: a tone selector, a cumulative loss estimator and a shift register. The tone selector selects a subset of the number of sub-channels within the bandwidth of a received wireline communication for which subset the sum of the square roots of the corresponding sub-channel indices may be expressed as an integer power xe2x80x9cnxe2x80x9d of 2. The cumulative loss estimator cumulates the insertion losses across the selected subset of the sub-channels and outputs a binary value corresponding thereto. The shift register couples to the cumulative loss estimator to scale a magnitude of the binary value received from the cumulative loss estimator by an amount proportionate to xe2x80x9cnxe2x80x9d to obtain a scaled electrical distance estimate for the wireline.
In an alternate embodiment of the invention a method for wireline characterization is disclosed. The method comprises:
selecting a subset of the number of sub-channels within the bandwidth of a received wireline communication for which subset the sum of the square roots of the corresponding sub-channel indices may be expressed as an integer power xe2x80x9cnxe2x80x9d of 2;
cumulating the insertion losses across the selected subset of the sub-channels and outputting a binary value corresponding thereto; and
scaling a magnitude of the binary value cumulated in said act of cumulating by an amount proportionate to xe2x80x9cnxe2x80x9d to obtain a scaled electrical distance estimate for the wireline.
A corresponding means is also disclosed.